By convention, single strands of DNA and RNA sequences are written in a 5′-to-3′ direction except as needed to illustrate the pattern of base pairing. 5′-end The 5′-end (pronounced "five prime end") designates the end of the DNA or RNA strand that has the fifth carbon in the sugar-ring of the deoxyribose or ribose at its terminus. A phosphate group attached to the 5′-end permits ligation of two nucleotides, i.e., the covalent binding of a 5′-phosphate to the 3′-hydroxyl group of another nucleotide, to form a phosphodiester bond. Removal of the 5′-phosphate prevents ligation. To prevent unwanted nucleic acid ligation (e.g. self-ligation of a plasmid vector in DNA cloning), molecular biologists commonly remove the 5′-phosphate with a phosphatase. The 5′-end of nascent messenger RNA is the site at which post-transcriptional capping occurs, a process which is vital to producing mature messenger RNA. Capping increases the stability of the messenger RNA while it undergoes translation, providing resistance to the degradative effects of exonucleases. It consists of a methylated nucleotide (methylguanosine) attached to the messenger RNA in a rare 5′- to 5′-triphosphate linkage. The 5′-flanking region of a gene often denotes a region of DNA which is not transcribed into RNA. The 5′-flanking region contains the gene promoter, and may also contain enhancers or other protein binding sites. The 5′-untranslated region (5′-UTR) is a region of a gene which is transcribed into mRNA, and is located at the 5′-end of the mRNA. This region of an mRNA may or may not be translated, but is usually involved in the regulation of translation. The 5′-untranslated region is the portion of the DNA starting from the cap site and extending to the base just before the AUG translation initiation codon of the main coding sequence. This region may have sequences, such as the ribosome binding site and Kozak sequence, which determine the translation efficiency of the mRNA, or which may affect the stability of the mRNA. 3′-end The 3′-end (three prime end) of a strand is so named due to it terminating at the hydroxyl group of the third carbon in the sugar-ring, and is known as the tail end. The 3′-hydroxyl is necessary in the synthesis of new nucleic acid molecules as it is ligated (joined) to the 5′-phosphate of a separate nucleotide, allowing the formation of strands of linked nucleotides. Molecular biologists can use nucleotides that lack a 3′-hydroxyl (dideoxyribonucleotides) to interrupt